1. Field of the Invention
This invention relates generally to composite articles of metal and ceramic and to methods of forming such articles. Specifically, the invention relates to composite metal and ceramic articles useful as heat resistant structures in internal combustion engines.
2. Background Art
The desirability of combining the heat resistance of ceramic with the workability and durability of a metal has long been recognized. For example, a variety of efforts have been directed in the past to forming a ceramic layer on the surface of metallic parts used in internal combustion engines. One of the first attempts to achieve a ceramic coated automotive part is described in U.S. Pat. Nos. 1,462,655 and 1,490,849 to Philip wherein a ceramic disc is entrapped within a metallic cap to form a piston. The cap and disc are then placed in a mold which is filled with molten iron or other metal so that the iron adheres to the metallic cap placed around the ceramic disc. It was found that the ceramic disc did not absorb heat and therefore the collection of hydrocarbon particles on the piston was decreased. U.S. Pat. Nos. 3,777,722, 4,142,500 and 2,657,961 also suggest ceramic coated metals for use in automotive applications.
Still others have disclosed methods for producing engine parts wherein ceramic particles are forced into the surface of a heated metallic automotive part. The result is a superficial ceramic-metal surface which partially insulates the adjacent all metallic portion, as described in U.S. Pat. Nos. 2,075,388 and 3,149,409. While these patents evidence a significant advance in the art, applicant has recognized that the bonding of the ceramic particles to the metal part is less than optimal in a number of aspects. Firstly, the integrity of the bond is questionable in that the ceramic particles may tend to coalesce, overlap, or clump together when injected into the molten metal. Since the coalesced particles are not totally surrounded by metal, the strength of the metal to ceramic bond is diminished such that the coalesced particles may break loose resulting in surface scalling, cracking, or pitting especially when exposed to high temperatures. Secondly, the resulting composite surface is partially made up of metal and partially of ceramic so that a heat transfer path to the metallic part still exists from the high temperature environment. Since the exposed metallic portion conducts heat quickly to the remainder of the part, the full benefits from combining the ceramic and metallic portions are not fully achieved.